Transmission devices known from practice, in particular automatic transmissions, are made with a hydraulic system comprising a main transmission pump for the supply of hydraulic fluid to various transmission components. Such main transmission pumps are often arranged on the transmission input side and can be driven by a torque applied to a transmission input shaft, for example by torque from a drive machine of a vehicle drivetrain. A force flow can be produced between the transmission input shaft and a transmission output shaft by various respective hydraulically actuated transmission components, this force flow being interrupted when the hydraulic system is not supplied by the main transmission pump device.
Vehicles made with transmission devices whose main transmission pumps can be driven by the engine are fundamentally characterized, as a consequence of the system, by reduced ability to be towed. Without special measures both the towing speeds and the towing distances are usually very limited, but this is undesirable.
The limited towing ability is the result of insufficient supply of the transmission components with hydraulic fluid by the main transmission pump when the drive machine is switched off, since during a towing process, while the drive wheels are in contact with the ground, a torque is transferred into the transmission device via a transmission output shaft and transmission components actively connected to the transmission output shaft are driven in rotation. Owing to the deficient lubrication and cooling of these driven transmission components, the frictional heat generated in the area of the transmission components during a towing process can only be dissipated to a limited extent, so that the thermal loading of the transmission components becomes undesirably high and sometimes seizing that permanently compromises the operation of the transmission device can take place.
The problems that result from inadequate supply of hydraulic fluid to a transmission device can sometimes also occur during other operating condition variations of a vehicle, depending on the design of the vehicle's drivetrain. For example, a transmission device as described earlier in a hybrid all-wheel-drive vehicle may likewise not be supplied to the desired extent with hydraulic fluid from a main transmission pump device arranged on the transmission input side, during purely electric driving operation by an electric machine which is directly associated with a vehicle axle without having to pass the drive torque of the electric machine via the transmission device.
Besides the main or primary pumps provided for supplying oil to an automatic transmission and driven by the engine, to avoid the above-mentioned problems additional, so-termed secondary or auxiliary pump devices are provided in transmissions, which supply a hydraulic system with hydraulic fluid in place of, or in addition to the main transmission pump, carry out other oil supply functions, and have separate driving means.
Disadvantageously, compared with transmission devices made without an auxiliary pump device, the last-mentioned design of transmission devices with a respective main transmission pump that can be driven by a drive engine and an auxiliary pump preferably driven by an electric motor increases the production costs of a transmission device.
Furthermore, in unfavorable operating situations the current uptake of an active auxiliary pump device places an undesirably high load on the vehicle's electric system. In addition, the arrangement of the auxiliary pump preferably on the outside of the housing of the transmission device takes up structural space in the vehicle and entails construction costs and effort for the hydraulic connection of the auxiliary pump to a hydraulic system of the transmission device and for the electrical connection of the auxiliary pump to the electric control and regulating system of a drivetrain of a vehicle, to an undesired extent.
Alternatively to the auxiliary pump device it is also possible to make vehicle drivetrains in which an active connection between the transmission and the drive output can be interrupted when necessary by an operator, in order to avoid operating conditions of transmission devices during which high loads occur without corresponding lubrication and cooling. The active connection between the transmission and the drive output can be broken for example by disconnecting the cardan shaft or by some other suitable measure, but vehicle drivetrains made in this way are again characterized by high production costs and are of complex design.